Biodiesel is being used as an alternative or supplement to petroleum-derived diesel fuel. Biodiesel can be made from various bio-generated oils and fats from vegetable and animal sources. One process involves the transesterification of triglycerides in the oils or fats with a lower alkanol in the presence of a base catalyst to produce alkyl ester and a glycerin co-product.
Biodiesel must meet demanding product specifications. See, for instance, ASTM D 6751, American Society for Testing and Materials. This standard provides, among other things, that biodiesel have a methanol content of less than 0.2 volume percent, a water and sediment content of no more than 0.05 volume percent, an acid number of no more than 0.50 milligrams of potassium hydroxide per gram of biodiesel, free glycerin of no more than 0.020 mass percent, and a phosphorus content of no more than 0.001 mass percent (calculated as elemental phosphorus). Biodiesel should also exhibit stability in storage, particularly stability against oxidation degradation.
Accordingly, refining of crude biodiesel is required to remove lower alkanol, water, and salts of fatty acids. Under the basic transesterification conditions, any free fatty acids contained in the raw material feed or generated during the process are converted to soaps. These soaps must be removed from the biodiesel product. A significant portion of the soaps can be removed by a phase separation of a glycerin-containing phase, which will also contain soaps, from the crude alkyl ester-containing biodiesel, and water washing which also serves to remove neutralized base catalyst.
To minimize the formation of soaps which must be removed from the crude biodiesel, many process operators prefer to use raw material feedstocks with a low content of free fatty acids such as refined soybean oil. However, the sought refined oils with low free fatty acid content may not be readily available, or other oils and fats may be more economically attractive raw material feedstocks. Hence, processes are sought that offer the ability to use raw material feedstocks containing free fatty acids, and especially processes that can tolerate changes in free fatty acid content of the raw material feedstocks.
One of the attractive features of processes for making biodiesel is that biodiesel processes do not require the complex feedstock treatment and distillation operations associated with a petroleum refinery. Consequently relatively small facilities to serve a local area can be economically viable in comparison to large, petroleum refinery-scale facilities. This economic viability would be reduced if substantial additional unit operations were required to handle free fatty acids and other undesirable impurities.
The unit operations for removal of soaps from crude biodiesel are sized to handle certain amounts of soaps. Thus the ability of the operator to use raw material feedstocks containing higher free fatty acid content is limited. Water presence during the transesterification is available for reaction with glycerides and alkyl ester to form free fatty acid which in turn reacts with base to form soaps. Additionally, the amount of soaps generated as side products during the transesterification process often increases with increased severity of the base-catalyzed transesterification reaction. Not only can the soaps provide processing difficulties in the transesterification process but also the capacity of the unit operations for removal of soaps can be limiting on production capacity.
Free fatty acids, while not directly useful in biodiesel as product specifications typically mandate that the biodiesel contain little free acid, can be converted to esters suitable for inclusion in biodiesel. Numerous processes have been proposed. See, for instance, U.S. Pat. No. 6,822,105; U.S. Patent Application Publication No. 2005/0204612; and Canakci, et al., Transactions of ASAE, 42, 5, pp. 1203-10 (1999).
Turck in U.S. Pat. No. 6,538,146 discloses a method for producing fatty acid esters of alkyl alcohols using oils that contain free fatty acids and phosphatides. They summarize their process as treating the feed with a base mixture of glycerin and a catalyst to produce a two phase mixture with the neutralized free fatty acids passing into the glycerin phase. The oil phase containing the triglycerides is then subjected to transesterification. See column 2, lines 35 et seq. At column 4, lines 41 et seq., Turck poses that the free fatty acids can be separated per WO 95/02661 and subject them to esterification with an alcohol. The esterified product can be added to the transesterification mixture.
Koncar, et al., in U.S. Pat. No. 6,696,583 disclose methods for preparing fatty acid alkyl esters in which fatty acids contained in a glycerin phase from a transesterification are separated and mixed with an esterification mixture containing triglycerides and is subjected to esterification to form fatty acid esters. The object of their process is to process the fatty acid phase in the untreated state, i.e., without purification and removal of sulfuric acid. The esterification product is then transesterified with alcohol. Koncar, et al, refer to EP-A-0 708 813 as disclosing the esterification of free fatty acids at column 2, lines 26 to 34.
Demmering, et al., in U.S. Pat. No. 5,773,636 disclose processes for the production of fatty acid lower alkyl esters in which the feed is treated with acid at elevated temperatures and then transesterified with lower alkanol. The patentees state that the acid treatment provides fatty acid lower alkyl ester free from unpleasant odors and discoloration.
Various processes are commercially offered for pretreating fats and oils to provide triglyceride-containing feeds for base transesterification. These processes remove free fatty acids and many are adapted to remove phospholipids. Westfalia Separator Food Tec GmbH offers a slate of pretreatment equipment and processes depending upon the sought product and the nature of the feed. In one process, herein referred to as the Alkaline Refining Process, feed is treated with acid to degum (removal of phospholipids), then contacted with base such as sodium hydroxide to convert free fatty acids to salts. The salts are removed by centrifuging and the feed is water washed and dried to provide a feedstock for transesterification. They also have an alcohol neutralization refining process. In this process, the feed is contacted with an acid/methanol solution, then contacted with an alkaline/methanol solution followed by contact with a glycerin/alcohol/alkali mixture from transesterification. Soaps are removed by centrifuge and the neutralized and degummed oil is fed to a base transesterification. Westfalia also offers a special degumming and alcohol neutralization for rapeseed oil, soybean oil and palm oil and a cold degumming plus alcohol neutralization process. These processes are described by Harten, Practical Short Course “Biodiesel”, Quebec Canada, May 2007.
Desmet Ballestera have a pretreatment process which involves an acid conditioning using phosphoric or citric acid to convert non-hydratable phosphorus to a hydratable form so that it can be removed with water. The resulting acid conditioned feed is neutralized with sodium hydroxide. Then soaps are removed by centrifuging. If necessary a silica purification step can be used. The feed is then dried. Bleaching can be done, but usually is not necessary. In their transesterification process, three reactors are used. Glycerin and catalyst is recovered from the second reactor and is fed along with methanol, additional catalyst and feed, to the first reactor. Kemper, Desmet Ballestra Biodiesel Production Technology, Biodiesel Short Course, Quebec City, Canada, May 12-13, 2007.
Crown Iron Works Company has a process for degumming and refining feeds involving acid treatment, followed by neutralization with caustic, centrifuging to remove soap or gums, silica treatment and then drying and filtering to provide a refined feed. Waranica, Crown lion Works Biodiesel Production Technology, Biodiesel Short Course, Quebec Canada.
Degumming, however, can result in a loss of feedstock that can be converted to biodiesel. Some feedstock such as crude soy oil can contain 3 mass percent or more of phospholipids. Most phospholipids are diglycerides with one substituent on the glycerin backbone being a phosphorus-containing moiety.
Another concern in producing biodiesel by transesterification of glycerides with lower alkanol is that the reaction, being equilibrium limited, must use a stoichiometric excess of lower alkanol to drive the transesterification to high conversions. In some instances, the amount of methanol provided is at least twice that required for stoichiometric transesterification. The unreacted lower alkanol preferentially partitions to glycerin and should, for an economically attractive process, be recovered from the glycerin and recycled to the transesterification. Processes such as described by Desmet Ballestra and Crown Iron Works distill methanol from the glycerin removed from the oil phase containing the alkyl esters for biodiesel. Water contained in the glycerin will also be removed during the distillation. As water reacts during transesterification to form free fatty acids and thus soaps, additional distillation may be required to dehydrate the methanol. The heat energy requirements for the distillation can represent a significant cost to the biodiesel producer.
The biodiesel producer can operate with a lower methanol to feed ratio and thereby reduce the amount of methanol that needs to be distilled from the glycerin per unit of biodiesel produced. The consequences of using lower methanol to triglyceride molar ratios is one or more of slower reaction rates and less complete conversion of the triglyceride to alkyl ester.
Processes are sought that are sufficiently robust to allow feeds of varying free fatty acid content to be used without undue complexities and capacity restrictions. Processes are further sought that can enable de-bottlenecking of existing, base-catalyzed transesterification processes. Processes are further sought that reduce the variable costs in making biodiesel including reducing the amounts of lower alkanol and base catalyst required per unit of biodiesel production and reducing the energy consumption of a facility making biodiesel.